DC-DC switching converters, i.e. devices that accept a DC input voltage and produce a DC output voltage, in particular on another voltage level, have a wide range of applications in today's power electronics. DC-DC Switching converters are used e.g. for power supplies, DC motor control or battery management. Apart from converting the input DC voltage they provide noise isolation, power bus isolation etc. The switching regulators allow for step-up operation or voltage inversion and offer a higher efficiency compared to linear regulators. Using a transformer as the energy-storage element also allows the output voltage to be electrically isolated from the input voltage.
However, the switching operation of the converter creates noise that has to be suppressed in order to avoid electromagnetic interference (EMI) affecting other devices connected to the power converter. To achieve this, an EMI filter, i.e. a suitable low-pass filter, is commonly arranged on the input side of the converter. The EMI filter rejects the fixed frequency current ripple generated by the switching converter.
In cases where more power has to be delivered into different loads or where different output voltages have to be provided it is known to parallel a plurality of DC-DC converters. The converters share the same input bus and use one common EMI filter. However, this arrangement leads to frequency beating phenomena between different converters sharing the same line input as well as to additional low frequency interferences. Altogether, the peak to peak amplitude of the interference may be about five times higher than those of just the input current ripple created by a single converter. This greatly increases the performance demand for the system EMI filter. In principle, most of the additional interference due to the parallel arrangement of the converters might be avoided by externally synchronizing the converter's switching frequencies.
However, synchronization is not easily possible with basic types of switching converters that are widely used for output voltages of several volts and output currents of about 10-60 A, namely the so-called “quarter-brick” or “eighth-brick” converters. These types have predefined mechanical dimensions and feature an industry-standard pin out, including two input and two output connectors, an On/Off connector for remotely controlling the device, two output sensing connectors for line drop compensation as well as a trim connector (called sometimes the “trim pin” or “trim terminal”) for adjusting the output voltage of the device.
However, the industry standard quarter and eighth-brick formats do not allow for an extra pin available for external frequency synchronization. Therefore, a sophisticated EMI filter device has to be employed with prior art quarter or eighth-brick converters, or the DC-DC converting circuit has to be completely redesigned in order to utilize specific DC-DC converters that provide extra pins for synchronization. Both possibilities are costly and cumbersome.